1. Field of the Invention
The present invention relates to apparatus for mooring a floating vessel in open sea, using a semi-submersible floating dock.
2. General Background and State of the Art
There is a fast growing demand for Liquid Natural Gas (LNG) in developed countries and as a result there is an increased need to import LNG into these countries.
Unfortunately, due to the nature of LNG as a cryogenic fluid, i.e. a gas in a cooled liquid form, it is perceived that there are a number of risks associated with its handling. For this reason, it is often difficult to obtain permission for the construction and operation of LNG receiving terminals, particularly in areas that may be densely populated, either at the shore or in harbours.
Alternatively, the LNG receiving terminals can be located offshore, away from any populated areas. However, transferring LNG between two offshore structures can pose a number of significant technical difficulties due to the large relative motions that may result between the vessels as a result of wave action acting upon them. Current offloading apparatus does little to reduce the effects of wave action upon two offshore vessels and consequently there is a need for improved fluid transfer apparatus.
It is known to use a submersible dock to transfer fluid from a first vessel to a receiving terminal. Typical arrangements of this sort are disclosed in patent documents GB 2,056,391, U.S. Pat. No. 3,841,501 and FR 1,421,700. However, such arrangements invariably suffer from operational disadvantages.
GB 2,056,391 discloses a submersible dock comprising a frame which is connected via a rigid articulated arm to an anchoring member on the seabed. Being rigidly connected to the sea bed the frame has limited movement in a vertical direction. Consequently, it would be unsuitable for mooring a tanker in heavy seas.
Conversely, U.S. Pat. No. 3,841,501 discloses a submersible dock having a range of movement limited only by the length of the fluid supply line. There are no integral means for mooring the tanker, other than to the submersible dock itself, and so the tanker must instead be moored by attachment to a separate buoy or submerged buoyant body.
There is therefore a need for a loading dock which can overcome these disadvantages and which is able to rigidly moor a vessel yet permit sufficient motion of the mooring means such that fluid transfer between the vessel and the receiving terminal can occur in heavy seas.